For commercially available rotation number detectors, there are conventional techniques described in Patent Literature 1 and Patent Literature 2 of such detectors that use self power-generation provided by a magnetic wire and a pickup coil. In Patent Literature 1, by using a power generating device that combines a magnet magnetized with one north pole and one south pole with a pickup coil and a magnetic wire that exhibits a large Barkhausen Effect, a back-up electric power source for the detector is supplied by the electrical power from the power generating device. The generator elements are 90 degrees out of phase for each revolution and two power generating devices are provided
In Patent Literature 2, a plurality of combinations of a magnetic wire and a coil are arranged radially, and changes in the inductance thereof are converted into electric signals.
The large Barkhausen effect is a phenomenon in which magnetizations are abruptly reversed concurrently in a magnet wire near a boundary of the S/N poles of a magnet. By detecting the magnetization reversal with a pickup coil, constant power generation pulses can be acquired regardless of the rotation speed of the object whose rotations are being detected, such as a motor.